Title: DFT+U-D3 study on the thermodynamics and modulation mechanism of H2O and CO2 adsorption on PuO2 surface

Abstract

The adsorption thermodynamics and modulation of active molecules on PuO2 surface are very important for the long-term storage of Pu. Within DFT+U-D3 scheme, a comparative study is performed to investigate the adsorption mechanisms, morphology, and phase diagrams of H2O and CO2 on PuO2 (111) surface. Unlike the multilayer BET model of H2O adsorption, CO2 adsorption accords with monolayer Freundlich model as revealed by the calculated isotherms. The different adsorption behaviors are mainly due to the hydrogen bonds and Coulomb interactions between the adsorbed molecules. The hydrogen bonds promote the polymolecular adsorption of H2O and the further partial dissociation at high coverage. The Coulomb attractive force among H2O is larger than CO2, and the attractive force among CO2rapidly decreases as the coverage increases. We clarifies the adsorption behaviors of H2O and CO2, and it could provide some important insights into the prediction of the surface corrosion reactions of PuO2 in storage containers. The polarons induced by interactions between excess charges and lattice distortions play an important role in PuO2surface chemistry. Our result reveals that surface and subsurface electron/hole polarons play different roles to H2O and CO2 adsorption. Surface polarons can promote H2O and CO2¬ adsorptions. It is interesting that the subsurface electron polarons can promote H2O adsorption but impede CO2 adsorption, whereas the subsurface hole polarons just play the opposite role. Base on the comparative analysis of the chemisorption properties of H2O and CO2, and the electronic structures of the polarons, we put forward the inverse modulation mechanism by subsurface electron and hole polarons on H2O and CO2 adsorption, which can be used to manipulate the active molecules’ chemisorption on oxides surface.